The invention relates to an echo compensation device, which has a digital filter in the form of a digital transverse filter having one input, for an information transmission system. It relates in particular to a hybrid circuit--also termed a 4-wire/2-wire interface--having such an echo compensation device.
The general problems of such hybrid circuits are described in the book "ADAPTIVE SIGNAL PROCESSING" by B. Widrow and S. D. Stearns, 1985 Prentice Hall, Englewood Cliffs, N.J., U.S.A., on pages 338 to 347 in conjunction with pages 15 to 29. In particular, FIG. 12.30 on page 344 shows a block diagram of an information transmission system having an adaptive echo compensation device, of which the signal characteristic model is represented in FIG. 12.31 on page 345 and which is described on pages 344 to 346. A suitable FIR filter (Finity Impulse Response filter) is shown as an adaptive transverse filter having one input in FIG. 2.2 on page 17 and described on pages 16 and 17. In the case of such a filter, an input signal is led via an iterative network of delay elements having a delay factor of the same size, the output signal of one delay element being provided in each case as the input signal for the following delay element. Both the input signal of the overall circuits and the output signals of all the delay elements are respectively weighted with a coefficient, and these weighted values are added to form a joint output signal. In the case of a hybrid circuit having an adaptive filter, the coefficients are automatically adapted during operation to the respective operating conditions, in particular to the line length and the reflection factor of the system. Less convenient hybrid circuits, which in the case of use in normally unchangeable network regions guarantee an adaptation of the input impedance of the hybrid circuit to the line to a sufficient extent, use filters having fixed coefficients tuned to the line. Fundamentals of suitable adaptation algorithms are explained on pages 19 ff. of the abovementioned book. Particularly favorably suited is a so-called MMSE method (Minimum Mean-Square Error, also LMS for Least Mean-Square), which takes account of the minimum mean-square error.
Special problems of hybrid circuits having an echo compensation device and their solution are treated in DE-C 31 41 502 (U.S. Pat. No. 4,381,561), U.S. Pat. No. 3,633,105, U.S. Pat. No. 3,798,560, EP-A 0 448 753 (U.S. Pat. No. 5,175,763) and EP-A 0 448 754 (U.S. Pat. No. 5,172,411). An embodiment of such a 2/4-wire hybrid circuit and of its mode of operation, is described in the article "A Signal-Processing Codec Filter for PCM Applications" by D. Vogel, E. Schmid, J. Reisinger and L. Lerach, published in Siemens Forschungs-- und Entwicklungsberichte 15 (1985) No. 5, Berlin, W. Germany on pages 253 to 258. This article treats a hybrid circuit whose echo compensation device contains an adaptive FIR filter which with the aid of the Leaky-LMS adaptation algorithm balances the echo path on the 4/2-wire interface in order thereby to extinguish the echo occurring on the hybrid.
U.S. Pat. No. 4,064,379 describes an echo compensation device for a hybrid circuit in which analog input signal are sampled in time-discrete fashion and are convened into a digital code using the A law. The sample values and the coefficients are stored in shift registers and multiplied in logarithmic form by addition in a summing device to form an output signal value.
It is common to all the previously described hybrid circuits that computational inaccuracies occur, owing to word length restriction, in the signal processor required to realize the filter. Because of such computational inaccuracies, the echo compensation device respectively used cannot operate with quiet input signals or low input levels.